Japanese Patent Application Publication No. H6-101534 A describes an evaporated fuel processing device. This evaporated fuel processing device includes a sensor configured to specify a fluid density of air introduced to a canister and a sensor configured to specify a fluid density of a gas mixture of evaporated fuel and air (which is hereinbelow termed “purge gas”) sent from the canister to an engine. The sensor for specifying the fluid density of the purge gas is disposed between the canister and an intake passage to the engine. A control valve configured to control supply of the purge gas to the intake passage is disposed between the canister and the intake passage. The evaporated fuel processing device uses the fluid density of air and the fluid density of the purge gas specified respectively by the two sensors while the purge gas is being supplied to the intake passage from the canister to calculate a concentration of the purge gas based on a ratio or a difference of those fluid densities.